In excess of 100,000 new golf cars are manufactured annually. Approximately 60% of the new golf cars are propelled by direct current (DC) series wound electric motors. The remaining 40% of the new golf cars are propelled by gasoline or propane fueled internal combustion engines.
DC electric motors are the preferred means to provide the motive force for golf cars, primarily because of reduced maintenance costs and least damage to the environment. Furthermore, substantially all of the electric golf cars use series wound DC motors. The inherent characteristics of a series wound DC motor make it ideal for use with a golf car. The inherent characteristics include high torque at low speeds for hill climbing and high speed at low torque for level ground.
Downhill braking action is a necessity for golf cars on hills to prevent dangerous runaway in the event of brake failure. The internal combustion engine provides inherent engine-braking action upon release of the throttle, especially with a built-in governor as used in golf cars. Series wound DC motors, on the other hand, provide no downhill braking action and can run away going downhill. As a result, operators of hilly golf courses are often forced to use the less desirable gasoline or propane fueled internal combustion engines in golf cars for safety reasons. Therefore, it would be desirable to have a DC electric motor with the characteristics of a series wound DC motor which provides downhill braking action.
Manufacturers of electric golf cars have devised several arrangements to provide electrical braking. The most common arrangement is to provide means to `plug-brake` or energize the motor to operate as a motor in the reverse direction of rotation while going forward downhill. In the event of a brake failure, the golf car driver must remember to throw a direction switch into reverse, and to carefully control the throttle to prevent injurious decelerating forces.
The majority of the electric golf cars produced today use electronic speed controllers. These controllers have included special circuitry to control the decelerating forces during the `plug-braking` operation, but the golf car driver must still have the presence of mind to throw the direction switch into reverse. Additionally, `plug-braking` depletes the battery and can overheat the electric motor.
To avoid the problems associated with `plug-braking` in series would DC motors, the use of electrical generator action for electrical braking has been suggested. However, conventional series wound DC electrical motors cannot make the transition from motor operation to generator operation. In the transition from motor operation to generator operation, the armature current must reverse direction while the field current must continue to flow in the original direction. The nature of the arrangement of the parts of a series wound DC motor prevents the current from going in two different directions in the same series connected circuit.
To overcome the natural limitations of the series wound DC motor, switching mechanisms reconnect the series wound DC motor as a shunt wound configuration. The switching mechanisms can be operated to reconnect the electromagnetic field windings in parallel with the armature windings so that the machine can operate as a shunt wound DC generator to provide electrical braking. These motor-to-generator schemes utilizing the switching mechanisms require large power contactors to accomplish the reconnection. The power contactors are generally too expensive for use in mass produced electric golf cars. Additionally, a malfunction of one of the contactors in the switching circuitry can cause the golf car to uncontrollably run away.
Therefore, it would be highly desirable to have a new and improved DC electrical motor system which converts from operation as a series wound DC motor to operation as a shunt wound DC generator, and vice versa, without utilizing large power contactors. Such an electrical motor system should be relatively inexpensive to manufacture.